Search

We searched for long-duration microlensing events originating from intermediate-mass black holes (BH) in the halo of the Milky Way, using archival data from the EROS-2 and MACHO photometric surveys towards the Large Magellanic Cloud (LMC). We combined data from these two surveys to create a common database of light curves for 14.1 million objects in the LMC, covering a total duration of 10.6 years, with flux series measured in four wide passbands. We carried out a microlensing search on these light curves, complemented by the light curves of 22.7 million objects, observed only by EROS-2 or only by MACHO, over about 7 years, with flux series measured in only two passbands. A likelihood analysis, taking into account the LMC self-lensing and Milky Way disk contributions, allows us to conclude that compact objects with masses in the range 10 − 100 M⊙ cannot make up more than ∼15% of a standard halo total mass (at a 95% confidence level). Our analysis sensitivity weakens for heavier objects, although we still rule out the possibility of ∼50% of the halo being made of ∼1000 M⊙ BHs. Combined with previous EROS results, an upper limit of ∼15% of the total halo mass can be obtained for the contribution of compact halo objects in the mass range 10−6 − 102 M⊙.

Galactic microlensing datasets now comprise in excess of $10^4$ events, and with the advent of next generation microlensing surveys that may be undertaken with facilities such as the Rubin Observatory (formerly LSST) and Roman Space Telescope (formerly WFIRST), this number will increase significantly. So too will the fraction of events with measurable higher order information such as finite source effects and lens-source relative proper motion. Analysing such data requires a more sophisticated Galactic microlens modeling approach. We present a new second-generation Manchester-Besan\c{c}on Microlensing Simulator (MaB$\mu$lS-2), which uses a version of the Besan\c{c}on population synthesis Galactic model that provides good agreement with stellar kinematics observed by HST towards the bulge. MaB$\mu$lS-2 provides high-fidelity signal-to-noise limited maps of the microlensing optical depth, rate and average timescale towards a 400 sq. degree region of the Galactic bulge in several optical to near-infrared pass-bands. The maps take full account of the unresolved stellar background as well as limb-darkened source profiles. Comparing MaB$\mu$lS-2 to the efficiency-corrected OGLE-IV 8,000 event sample shows a much improved agreement over the previous version of MaB$\mu$lS, and succeeds in matching even small-scale structural features in the OGLE-IV event rate map. However, there remains evidence for a small under-prediction in the event rate per source and over-prediction in timescale. MaB$\mu$lS-2 is available online () to provide on-the-fly maps for user supplied cuts in survey magnitude, event timescale and relative proper motion., Comment: 24 pages, 21 figures, Journal: MNRAS, for associated online tool, visit

We used Gaia Data Release 2 to search for upcoming photometric microlensing events, identifying two candidates with high amplification. In the case of candidate 1, a spectrum of the lens (l1) confirms it is a usdM3 subdwarf with mass $\approx 0.11 M_\odot$, while the event reaches maximum amplification of $20^{+20}_{-10}$ mmag on November 3rd 2019 ($\pm$1d). For candidate 2, the lens (l2) is a metal-poor M dwarf with mass $\approx 0.38 M_\odot$ derived from spectral energy distribution (SED) fitting, and maximum amplification of $10^{+40}_{-10}$ mmag occurs on June 3rd 2019 ($\pm$4d). This permits a new algorithm for mass inference on the microlens. Given the predicted time, the photometric lightcurve of these events can be densely sampled by ground-based telescopes. The lightcurve is a function of the unknown lens mass, together with 8 other parameters for all of which Gaia provides measurements and uncertainties. Leveraging this prior information on the source and lens provided by Gaia's astrometric solution, and assuming that a ground-based campaign can provide 50 measurements at mmag precision, we show for example that the mass of l1 can be recovered to within 20 per cent (68 per cent confidence limit)., MNRAS, submitted

The microlensing surveys MACHO, EROS, MOA and OGLE (hereafter called MEMO) have searched for microlensing toward the Large Magellanic Cloud for a cumulated duration of 27 years. We study the potential of joining these databases to search for very massive objects, that produce microlensing events with a duration of several years. We identified the overlaps between the different catalogs and compiled their time coverage to identify common regions where a joint microlensing detection algorithm can operate. We extrapolated a conservative global microlensing detection efficiency based on simple hypotheses, and estimated detection rates for multi-year duration events. Compared with the individual survey searches, we show that a combined search for long timescale microlensing should detect about ten more events caused by $100.M_{\odot}$ black holes if these objects have a major contribution to the Milky Way halo. Assuming that a common analysis is feasible, meaning that the difficulties that arise from using different passbands can be overcome, we show that the sensitivity of such an analysis might enable us to quantify the Galactic black hole component., Comment: 5 pages, 4 figures. Accepted for publication in Astronomy & Astrophysics

Time-delay strong lensing provides a unique way to directly measure the Hubble constant (H-0). The precision of the H-0 measurement depends on the uncertainties in the time-delay measurements, the mass distribution of the main deflector(s), and the mass distribution along the line of sight. Tie & Kochanek have proposed a new microlensing effect on time delays based on differential magnification of the coherent accretion disc variability of the lensed quasar. If real, this effect could significantly broaden the uncertainty on the time-delay measurements by up to 30 per cent for lens systems such as PG 1115+080, which have relatively short time delays and monitoring over several different epochs. In this paper we develop a new technique that uses the cosmological time-delay ratios and simulated microlensing maps within a Bayesian framework in order to limit the allowed combinations of microlensing delays and thus to lessen the uncertainties due to the proposed effect. We show that, under the assumption of Tie & Kochanek, the uncertainty on the time-delay distance (D-Delta t, which is proportional to 1/H-0) of the short time-delay (similar to 18 d) lens, PG 1115+080, increases from similar to 7 per cent to similar to 10 per cent by simultaneously fitting the three time-delay measurements from the three different data sets across 20 yr, while in the case of the long time-delay (similar to 90 d) lens, the microlensing effect on time delays is negligible as the uncertainty on D-Delta t of RXJ 1131-1231 only increases from similar to 2.5 per cent to similar to 2.6 per cent.

arXiv:1706.10281v2, A galaxy cluster acts as a cosmic telescope over background galaxies but also as a cosmic microscope magnifying the imperfections of the lens. The diverging magnification of lensing caustics enhances the microlensing effect of substructure present within the lensing mass. Fine-scale structure can be accessed as a moving background source brightens and disappears when crossing these caustics. The recent discovery of a distant lensed star near the Einstein radius of the galaxy cluster MACSJ1149.5+2223 allows a rare opportunity to reach subsolar-mass microlensing through a supercritical column of cluster matter. Here we compare these observations with high-resolution ray-tracing simulations that include stellar microlensing set by the observed intracluster starlight and also primordial black holes that may be responsible for the recently observed LIGO events. We explore different scenarios with microlenses from the intracluster medium and black holes, including primordial ones, and examine strategies to exploit these unique alignments. We find that the best constraints on the fraction of compact dark matter (DM) in the small-mass regime can be obtained in regions of the cluster where the intracluster medium plays a negligible role. This new lensing phenomenon should be widespread and can be detected within modest-redshift lensed galaxies so that the luminosity distance is not prohibitive for detecting individual magnified stars. High-cadence Hubble Space Telescope monitoring of several such optimal arcs will be rewarded by an unprecedented mass spectrum of compact objects that can contribute to uncovering the nature of DM., J.M.D. acknowledges the support of projects AYA2015-64508-P (MINECO/FEDER, UE), AYA2012-39475-C02-01, and the consolider project CSD2010-00064 funded by the Ministerio de Economia y Competitividad.M.O. is supported in part by World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan, and JSPS KAKENHI Grant Numbers 26800093 and 15H05892. M.J. acknowledges support by the Science and Technology Facilities Council (grant number ST/L00075X/1). Support for S.R. through HST programs GO-13386, GO14199, and GO-14208 was provided by NASA through grants from the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. A.V.F. and P.L.K. were supported by NASA/HST grants GO-14041, GO-14199, GO-14208, GO-14528, GO-14872, and GO-14922 from STScI; they are also grateful for financial assistance from the Christopher R. Redlich Fund, the TABASGO Foundation, and the Miller Institute for Basic Research in Science (U.C. Berkeley).

Microlensing is able to reveal multiple body systems located several kilo-parsec away from the Earth. Since it does not require the measurement of light from the lens, microlensing is sensitive to a range of objects from free-floating planets to stellar black holes. But if the lens emits enough light, the microlensing model predictions can be tested with high-resolution imaging and/or radial velocity methods. Such follow-up was done for the microlensing event OGLE-2011-BLG-0417, which was expected to be a close by ($\le$ 1 kpc), low-mass ($\sim 0.8 M_\odot$) binary star with a period $P\sim 1.4$ yr. The spectroscopic follow-up observations conducted with the VLT did not measure any variation in the radial velocity, in strong contradiction with the published microlensing model. In the present work, we remodel this event and find a simpler model in agreement with all the available measurements, including the recent GAIA DR2 parallax constraints. We also present a new way to distinguish degenerate models by using the GAIA DR2 proper motions. This work stresses the importance of thorough microlensing modeling, especially with the horizon of the {\it WFIRST} and the {\it Euclid} microlensing space missions., Comment: ApJ accepted

Proxima Centauri, our closest stellar neighbour, is a low-mass M5 dwarf orbiting in a triple system. An Earth-mass planet with an 11 day period has been discovered around this star. The star's mass has been estimated only indirectly using a mass-luminosity relation, meaning that large uncertainties affect our knowledge of its properties. To refine the mass estimate, an independent method has been proposed: gravitational microlensing. By taking advantage of the close passage of Proxima Cen in front of two background stars, it is possible to measure the astrometric shift caused by the microlensing effect due to these close encounters and estimate the gravitational mass of the lens (Proxima Cen). Microlensing events occurred in 2014 and 2016 with impact parameters, the closest approach of Proxima Cen to the background star, of 1\farcs6 $\pm$ 0\farcs1 and 0\farcs5 $\pm$ 0\farcs1, respectively. Accurate measurements of the positions of the background stars during the last two years have been obtained with HST/WFC3, and with VLT/SPHERE from the ground. The SPHERE campaign started on March 2015, and continued for more than two years, covering 9 epochs. The parameters of Proxima Centauri's motion on the sky, along with the pixel scale, true North, and centering of the instrument detector were readjusted for each epoch using the background stars visible in the IRDIS field of view. The experiment has been successful and the astrometric shift caused by the microlensing effect has been measured for the second event in 2016. We used this measurement to derive a mass of 0.150$^{\textrm{+}0.062}_{-0.051}$ (an error of $\sim$ 40\%) \MSun for Proxima Centauri acting as a lens. This is the first and the only currently possible measurement of the gravitational mass of Proxima Centauri., Comment: 10 pages, 6 figures, accepted by MNRAS

EROS has searched for microlensing toward four directions in the Galactic plane away from the Galactic center. The interpretation of the catalog optical depth is complicated by the spread of the source distance distribution. We compare the EROS microlensing observations with Galactic models, tuned to fit the EROS source catalogs, and take into account all observational data such as the microlensing optical depth, the Einstein crossing durations, and the color and magnitude distributions of the catalogued stars. We simulated EROS-like source catalogs using the Hipparcos database, the Galactic mass distribution, and an interstellar extinction table. Taking into account the EROS star detection efficiency, we were able to produce simulated color-magnitude diagrams that fit the observed diagrams. This allows us to estimate average microlensing optical depths and event durations that are directly comparable with the measured values. Both the Besancon model and our Galactic model allow us to fully understand the EROS color-magnitude data. The average optical depths and mean event durations calculated from these models are in reasonable agreement with the observations; consequently our simulation allows a better understanding of the lens and source spatial distributions in the microlensing events. Varying the Galactic structure parameters through simulation, we were also able to deduce contraints on the kinematics of the disk, the disk stellar mass function (IMF at a few kpc distance from the Sun), and the maximum contribution of a thick disk of compact objects in the Galactic plane (Mthick < 5 - 7 x 10**10 Msun at 95%, depending on the model). We also show that the microlensing data toward one of our monitored directions are significantly sensitive to the galactic bar parameters, although much larger statistics are needed to provide competitive constraints., 16 pages, 17 figures, submitted to A&A

International audience; Galactic bulge microlensing surveys provide a probe of Galactic structure. We present the first field-by-field comparison between microlensing observations and the Besançon population synthesis Galactic model. Using an updated version of the model we provide maps of optical depth, average event duration and event rate for resolved source populations and for difference imaging analysis (DIA) events. We also compare the predicted event time-scale distribution to that observed. The simulation follows the selection criteria of the MOA-II survey. We modify the Besançon model to include M dwarfs and brown dwarfs. Our best-fitting model requires a brown dwarf mass function slope of −0.4. The model provides good agreement with the observed average duration, and respectable consistency with the shape of the time-scale distribution (reduced χ2 ≃ 2.2). The DIA and resolved source limiting yields bracket the observed number of events by MOA-II (2.17 × and 0.83 × the number observed, respectively). We perform a two-dimensional fit to the event spatial distribution to predict the optical depth and event rate across the Galactic bulge. The most serious difficulty for the model is that it provides only ∼50 per cent of the measured optical depth and event rate per star at low Galactic latitude around the inner bulge (|b| < 3°). This discrepancy most likely is associated with known underestimated extinction and star counts in the innermost regions and therefore provides additional support for a missing inner stellar population.

We present the analysis of MOA-2007-BLG-197Lb, the first brown dwarf companion to a Sun-like star detected through gravitational microlensing. The event was alerted and followed-up photometrically by a network of telescopes from the PLANET, MOA, and uFUN collaborations, and observed at high angular resolution using the NaCo instrument at the VLT. From the modelling of the microlensing light curve, we derived the binary lens separation in Einstein radius units (s~1.13) and a mass ratio of (4.732+/-0.020)x10^{-2}. Annual parallax, lens orbital motion and finite source effects were included in the models. To recover the lens system's physical parameters, we combined the resulting light curve best-fit parameters with (J,H,Ks) magnitudes obtained with VLT NaCo and calibrated using IRSF and 2MASS data. We derived a lens total mass of 0.86+/-0.04 Msun and a lens distance of 4.2+/-0.3 kpc. We find that the companion of MOA-2007-BLG-197L is a brown dwarf of 41+/-2 Mjup observed at a projected separation of 4.3+/-0.1 AU, and orbits a 0.82+/-0.04 Msun G-K dwarf star. We study the statistical properties of this population of brown dwarfs detected by microlensing, transit, radial velocity, and direct imaging (most of these objects orbit solar-type stars), and we performed a two-dimensional, non-parametric probability density distribution fit to the data, which draws a structured brown dwarf landscape. We confirm the existence of a region that is strongly depleted in objects at short periods and intermediate masses (P500 d) and high masses (M>50 Mjup). While these data provide important clues on mechanisms of brown dwarfs formation, more data are needed to establish their relative importance, in particular as a function of host star mass., 16 pages, 12 figures

The microlensing event OGLE-2008-BLG-510 is characterised by an evident asymmetric shape of the peak, promptly detected by the ARTEMiS system in real time. The skewness of the light curve appears to be compatible both with binary-lens and binary-source models, including the possibility that the lens system consists of an M dwarf orbited by a brown dwarf. The detection of this microlensing anomaly and our analysis demonstrates that: 1) automated real-time detection of weak microlensing anomalies with immediate feedback is feasible, efficient, and sensitive, 2) rather common weak features intrinsically come with ambiguities that are not easily resolved from photometric light curves, 3) a modelling approach that finds all features of parameter space rather than just the `favourite model' is required, and 4) the data quality is most crucial, where systematics can be confused with real features, in particular small higher-order effects such as orbital motion signatures. It moreover becomes apparent that events with weak signatures are a silver mine for statistical studies, although not easy to exploit. Clues about the apparent paucity of both brown-dwarf companions and binary-source microlensing events might hide here., 17 pages with 8 figures, MNRAS submitted